Digital process for obtaining a measured parameter from an electric signal

ABSTRACT

A process for obtaining a measured value from an electric signal with a predetermined rated frequency, wherein the electric signal is scanned with a scanning frequency that is N times the rated frequency. After analog-to-digital conversion, the measured value is determined in an analyzer. In order to determine the measured value with a comparatively high accuracy, a measured value (f Nist ) is generated by means of a frequency meter (8). The measured value (f Nist ) is multiplied by a factor N yielding a derived measured value (f Asoll ). An analog-to-digital converter (1) is clocked with a scanning frequency (f A ) that corresponds to the derived measured value (f Asoll ).

FIELD OF THE INVENTION

The present invention relates to a digital process for obtaining ameasured parameter from an electric signal having a predetermined ratedfrequency, and more particularly wherein:

the electric signal is scanned by a scanning device that yields scanningvalues with a scanning frequency that is generated by a clock generatorand corresponds to N times the rated frequency of the electric signal,

the scanned values are converted to digital values in an analog-digitalconverter and

the measured value parameter is obtained from the digital values in ananalyzer.

BACKGROUND INFORMATION

A digital process for obtaining a measured parameter from an electricsignal is described, for example, in the book by P. Profos and T.Pfeifer, Handbuch der industriellen Messtechnik Handbook of IndustrialMeasurement Technology!, 1992, pages 404 and 405. In this known method,in order to measure the effective value, an electric signal formed froman electric voltage with a predetermined frequency is scanned andsubjected to an analog-to-digital conversion and then used to calculatethe effective voltage value in an analyzer. An effective current valueand the electric power can be determined with this known method.

Other known processes of this type are described in the book by R.Felderhoff Elektrische und elektronische Messtechnik Electric andElectronic Measurement Technology!, 5th edition, 1990, pages 307 to 311and the book by R. Lappe and F. Fischer Leistungselektronik-MesstechnikPower Electronic Measurement Technology!, 2nd edition, 1993, page 242.

The frequency measurement can be performed as described in the articleby J. Heydeman and E. N. Lulf "Microprocessor Based UnderfrequencyRelaying," Delft University of Technology, the Netherlands, published inIEE Conference Publication No. 249, Third Internal Conference onDevelopments in Power System Protection, 1985, pages 24 to 28. Thevoltage is scanned with a frequency of 2.5 kHz, which is accomplished byprogramming the microprocessor to generate interrupts with thisfrequency. The passage of the voltage through zero is detected by thesoftware. The frequency of the voltage can then be calculated from thenumber of pulses counted during one period of the voltage by forming theratio of the scanning frequency to the number of pulses per period. Theaccuracy of this known method of frequency measurement is determinedfirst by the accuracy that can be achieved in determining the length ofthe period and second by the scanning frequency.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a digital method ofdetermining a measured value from an electric signal, so the measuredvalue can be determined with a much greater accuracy than has beenpossible with the known method described above, and this can beaccomplished without any significant increase in effort or expense andwithout requiring a longer analysis time.

To achieve this object, the following measures are implemented accordingto the present invention in a digital process of the type describedabove:

a measured value that corresponds to the instantaneous frequency of theelectric signal is determined with a frequency measurement device,

the measured value is multiplied by a factor N, generating a derivedmeasured value,

an intermediate value that corresponds to the ratio of the clockfrequency of the clock generator to the derived measured value is formedin a quotient forming circuit and

this intermediate value is used to adjust the divider ratio of afrequency divider installed between the clock generator and the clockinput of an analog-digital converter so that the frequency correspondingto the derived measured value is sent to the clock input, where a changein the divider ratio is not implemented until after several periods ofthe electric signal have elapsed.

It is known from German Patent 41 22 399 A1 that the scanning frequencyin a process for digital measurement of voltages and currents in a powersupply system can be derived from the instantaneous frequency of thevariable parameter measured in order to be able to perform relativelyaccurate measurements, German Patent No. 41 22 399 A1, however, does notcontain any reference to a clock generator or a downstream quotientforming circuit that receives a measured parameter derived from theinstantaneous frequency of the electric signal measured.

An important advantage of the process according to the present inventionis that regardless of the instantaneous frequency of the analog electricsignal measured in each case, it uses a scanning frequency correspondingto N times the instantaneous frequency of the electric signal. Thisyields a great increase in measurement accuracy because the number ofscans performed per period of the electric signal is always the sameeven at signal frequencies other than the rated frequency. In theprocess according to the present invention, the scanning frequency isadjusted to the respective measured frequency of the electric signal.This is accomplished with a great accuracy because an intermediate valuethat corresponds to the ratio of the clock frequency of the clockgenerator to the derived measured value is formed in the quotientforming circuit. This assures that the scanning frequency will always bederived anew from the clock frequency of the clock generator. This isdone with a high accuracy because an intermediate value corresponding tothe ratio of the clock frequency of the clock generator and the derivedmeasured value is formed in the quotient forming circuit. This assuresthat the scanning frequency will always be derived anew from the clockfrequency of the clock generator.

In the process according to the present invention, the frequencymeasurement device is preferably a digital frequency meter because ofthe high accuracy it permits in measuring the frequency.

The frequency measurement can be performed as described in the articleby J. Heydeman and E. N. Lulf "Microprocessor Based UnderfrequencyRelaying", Delft University of Technology, the Netherlands, published inIEE Conference Publication No. 249, Third Internal Conference onDevelopments in Power System Protection, 1985, pages 24 to 28. Thevoltage is scanned with a frequency of 2.5 kHz, which is accomplished byprogramming the microprocessor to generate interrupts with thisfrequency. The passage of the voltage through zero is detected by thesoftware. The frequency of the voltage can then be calculated from thenumber of pulses counted during one period of the voltage by forming theratio of the scanning frequency to the number of pulses per period. Theaccuracy of this known method of frequency measurement is determinedfirst by the accuracy that can be achieved in determining the length ofthe period and second by the scanning frequency.

The frequency of the electric signal can be determined with anespecially high accuracy with a process whereby the digital frequencymeter, as a linear-phase nonrecursive digital filter that receives theelectric signal and has a symmetrical distribution of its filtercoefficients, is designed as an all-pass filter; another linear-phasenonrecursive digital filter that also receives the electric signal andhas a symmetrical distribution of its filter coefficients is designed asa low-pass filter such that its transmission function has a value of oneat a predetermined frequency of the electric signal. Downstream from theall-pass filter there is a circuit element having a linear-phasetransverse filter with a symmetrical distribution of its filtercoefficients at the input in one branch, a downstream delay element anda downstream multiplier, and at the input, in a branch parallel to thefirst branch, there is another linear-phase transverse filter with anantisymmetrical distribution of its filter coefficients and with atransmission function value of one at a predetermined frequency, anotherdownstream delay element and another downstream multiplier, where onemultiplier is also connected at the input to an output of the secondtransverse filter, and the other multiplier is also connected to theoutput of the first transverse filter, and it contains a differentiatingcircuit whose inputs are connected to the outputs of the multiplier. Acircuit device identical in design to the circuit element is connecteddownstream from the low-pass filter and the output of thedifferentiating circuit of the circuit element and the output of thecircuit device are connected to a quotient forming circuit, which isupstream from a square root extractor that is in turn connected to aninverse function forming circuit. Additional details and advantages ofthis type of frequency measurement can be obtained from the GermanPatent Application P 42 11 946.4 or the corresponding InternationalPatent Application PCT/DE/93/00262.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a system for carrying out the process ofthe present invention.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a system for implementing the presentinvention. The diagram of FIG. 1 serves primarily to illustrate theprocess according to the present invention. In practice, the processaccording to the present invention would preferably be carried out usinga microprocessor.

As FIG. 1 shows, an analog electric signal u, such as the voltage of anelectric power supply system (not shown), is sent to an analog-digitalconverter 1 that has an integrated scanning device 2 at the input. Ananalyzer 5 is connected downstream from analog-digital converter 1 todetermine the effective value u_(eff) of voltage u. In addition, ascanning signal generator 7 connected to the output of theanalog-digital converter 1 has a digital frequency meter 8 at its input.The scanned digital values of electric signal u are picked up bydownstream digital frequency meter 8 which may be designed and mayfunction as described in detail in the article mentioned above. However,an arrangement such as that disclosed in the German Patent Application P42 11 946.4-35 or the corresponding International Patent ApplicationPCT/DE/93/00262 is the preferred type of frequency meter. Digitalfrequency meter 8--as well as analyzer 5--has a clock generator 9 thatdelivers a clock frequency f_(Q) to the digital frequency meter 8. Ameasured value f_(Nist) that represents the instantaneous frequency ofthe signal u is then obtained at the output of digital frequency meter8.

The measured value f_(Nist) is sent to a multiplier 10 where it ismultiplied by a factor N. The factor N represents the ratio of thescanning frequency f_(A) to the rated frequency of the analog electricsignal u. For example, with a voltage in an electric power supply systemas the analog electric signal u, the rated frequency is 50 Hz. Thiscorresponds to the rated frequency of the system. A scanning frequencyf_(A) of 1 kHz, for example, is selected, so the factor N has a value of20.

A derived measured value f_(Asoll) that is obtained at the output ofmultiplier 10 provides the theoretical scanning frequency. In thenumerical example given above, this frequency is thus 1 kHz in the casewhen the voltage u has a frequency of exactly 50 Hz. However, if theinstantaneous frequency at the output of the digital frequencymeasurement device is found to be 49.5 Hz, for example, so that themeasured value f_(Nist) is 49.5 Hz, then a theoretical scanningfrequency of only 990 Hz is calculated, because the derived measuredvalue f_(Asoll) is equal to N×f_(Nist). The derived measured valuef_(Asoll) is sent to a downstream quotient forming circuit 11 that alsoreceives the clock frequency f_(Q) of clock generator 9. Then anintermediate value M that can be described by the following equation isobtained at the output of the quotient forming circuit:

    f.sub.Q /f.sub.Asoll =M

The intermediate value M is used to alter the divider ratio of adownstream frequency divider 12 whose other input is connected directlyto clock generator 9. Thus a signal with the frequency f_(Aist) that canbe described by the following equation is obtained at the output offrequency divider 12:

    f.sub.Aist =f.sub.Q ·(f.sub.Asoll /f.sub.Q)

Therefore, the frequency f_(Aist) corresponds to the theoreticalscanning frequency, which assures that the analog-digital converter 1will always be cycled with a frequency f_(A) that corresponds exactly toN times the respective instantaneous frequency of the electric signal u.The output of frequency divider 12 is connected to a clock input 13 ofanalog-digital converter 1.

To keep the operation of the process described here stable,analog-digital converter 1 does not receive a newly adjusted scanningfrequency immediately after there is a change in measured value f_(Nist)but instead a delay is implemented in a manner that is not shown herebut may be on the order of four periods of the electric signal u, forexample. Only after four periods of the signal u have elapsed does anychange in measured value f_(Nist) have an effect on scanning frequencyf_(A).

What is claimed is:
 1. A method for determining an effective value of anelectric signal with a predetermined rated frequency, wherein:theelectric signal is scanned and digitized in an analog-to-digitalconverter with a scanning device having a scanning frequencycorresponding to a factor N times the rated frequency of the electricsignal, the analog-to-digital converter generating digital sample valuesfrom the electric signal, and an analyzer is used to determine theeffective value from the digital sample values, the method comprisingthe steps of:measuring the digital sample values generated by theanalog-to-digital converter, with a frequency meter, to obtain ameasured value corresponding to an instantaneous frequency of theelectric signal; multiplying the measured value by the factor N to yielda derived measured value; generating an intermediate value Mcorresponding to a quotient of a clock frequency generated by a clockgenerator and the derived measured value; generating the scanningfrequency by dividing, in a frequency divider, the clock frequency by anadjustable divider ratio; and adjusting the divider ratio of thefrequency divider using the intermediate value M so that the scanningfrequency of the scanning device is equal to the derived measured value,wherein the step of adjusting the divider ratio is delayed for severalperiods of the electric signal.
 2. The method of claim 1, wherein thefrequency meter is a digital frequency meter.
 3. A system fordetermining an effective value of an electric signal with apredetermined rated frequency, wherein:the electric signal is scannedand digitized in an analog-to-digital converter with a scanning devicehaving a scanning frequency corresponding to a factor N times the ratedfrequency of the electric signal, the analog-to-digital convertergenerating digital sample values from the electric signal, and ananalyzer is used to determine the effective value from the digitalsample values, the system comprising:a frequency meter receiving thedigital sample values for generating a measured value corresponding toan instantaneous frequency of the electric signal; a multiplier formultiplying the measured value by the factor N to yield a derivedmeasured value; a quotient forming means for generating an intermediatevalue M corresponding to a quotient of a clock frequency generated by aclock generator and the derived measured value; a frequency divider forgenerating the scanning frequency by dividing the clock frequency by anadjustable divider ratio, wherein the divider ratio is adjusted inaccordance with the intermediate value M so that the scanning frequencyof the scanning device is equal to the derived measured value; and delaymeans for delaying the adjustment of the divider ratio for severalperiods of the electric signal.
 4. The system of claim 3, wherein thefrequency meter is a digital frequency meter.